Many useful applications, such as the detection of radioactive material and computer-assisted tomography (“CAT”), rely on the detection of photon radiation, known as X-ray and/or gamma-ray radiation. Both of these types of high-energy photon radiation cause ionization and for the purposes of this disclosure the two terms, X-ray and gamma-ray, are used interchangeably. In terms of the detection of such ionizing radiation, the spectral region of greatest interest for most applications generally falls between the energies of about 20 to 2,000 keV (i.e., 0.02 to 2 MeV).
In the above spectral range of interest, the primary types of interaction are the photoelectric and Compton effects. The relative contribution from each can be determined in quantitative fashion a priori via the combination of the incident photon energy and the atomic number (i.e., Z-number) of the interacting atom. The photoelectric effect describes a single atomic absorption, whereas the Compton effect describes an inelastic scattering collision that simultaneously results in a Compton recoil electron and a Compton scattered photon. The latter can be inelastically scattered again and again, until the photon either exits or is “absorbed” by the interacting media. Of the two processes, the primary basis for the majority of known ionizing radiation detectors used in imaging applications at photon energies up to at least 200 keV is the photoelectric effect, which causes the initial production of a single “free-electron” and a corresponding positive atomic ion.
In order to detect ionizing electromagnetic radiation, several known sensing devices are commonly used. One of the earliest known electronic devices is the ionization chamber. Detection of radiation in an ionization chamber, such as a Geiger-Müeller (“GM”) tube, is based upon electrical conductivity induced in an inert gas (usually containing argon and neon) as a consequence of ion-pair formation.
More recent and sensitive variations of sensing devices include high-pressure xenon ionization chambers, such as the tube disclosed in G. P. Lasche' et al., “Detection Sensitivity for Special Nuclear Materials with an Advanced High-Pressure Xenon Detector and Robust Fitting Analysis”, IEEE Trans. Nucl. Sci., 48 (2001), pp. 325-32, which is designed for portal inspection to detect the presence of 235U and 239PU radionuclides. However, these devices are very expensive and like a GM-tube have no internal pixel structure, so cannot count “simultaneous” electrons created in different spatial regions of the tube, nor be internally configured to function as an imaging detector.
In contrast, a gas proportional scintillation counter (“GSPC”) is an imaging device in which gaseous interaction primarily with low energy radiation in a high-voltage electric field causes secondary VUV photons that are detected by VUV-sensitive, photodiodes or photomultiplier (“PM”) tubes. Some disadvantages of GSPC's are their limited energy detection range, required use of ultrahigh-vacuum technology and ultrahigh-purity gases, and very short device lifetimes as measured in months.
For thermal (i.e., slow, 2200-m/s) neutron detection, the two-dimensional microstrip gas chamber (“MSGC”) is probably the most common known detector, although other related gas-based detectors such as multiwire proportional counters (“MWPC”), multitube position sensitive detectors (“PSD”), and GSPC's are also commonly used. However, these two-dimensional detectors are generally suitable only for large research laboratories that can support highly specialized detector groups, as they are often custom-built and can be difficult to maintain. In addition, they all require ultrahigh purity gas mixtures composed typically of 3He or 10BF3 as the neutron converter and CF4 as the quencher, and operate at positive pressures of about 3 to 20 atmospheres, presenting a potentially explosive hazard.
Several other known gas detector configurations have received considerable attention over the past few years including: gas electron multipliers (“GEM”), microgap chambers (“MGC”), and various combinations of MSGC and GEM detectors such as multiple-GEM, Micromegas and MICROMEGEM. The GEM structures operate in tandem with MSGC's to improve electron gain by charge pre-amplification.
The above-described gas detector configurations were developed primarily for use in detecting either low energy radiation (i.e. less than 10 keV), or very high energy particles (e.g., 300 MeV to 10 GeV pions) in particle accelerators. With regard to the various MSGC configurations (including the MSGC-GEM), they are designed to operate in the “Proportional Region” of the gas ionization curve, having a typical gas avalanche multiplication gain of ˜104.
Currently, the most effective radiation detector is generally considered to be a scintillation counter. The basic scintillation counter consists primarily of two components—a scintillation plate or crystal, optically coupled to a photomultiplier tube or a silicon photodiode. The scintillation plate or crystal contains phosphor type material that produces visible (or ultraviolet) photons upon the occurrence of an absorption/scattering event caused by incident ionizing radiation. Light from the scintillation material, which commonly is NaI(Tl), is transmitted to the photocathode of the photomultiplier, which, through a series of dynodes, amplifies the electrical signal.
Compared to a GM-tube that can have a “dead-time” on the order of 100 μs (microseconds) between counting events, during which time any response to radiation is impossible, a scintillation detector generally has a dead-time of about 1 μs or less. Another advantage of the scintillation detector is that the number of emitted photons produced by the scintillation plate or crystal, upon interaction with ionizing radiation, is approximately proportional to the energy of the incident radiation.
Further, for imaging applications, the scintillation counter is position-sensitive, and can yield good quality, medium resolution images. However, the resolution is limited by several factors, including the plate or crystal thickness, the photocathode spatial resolution, and the spatial separation between the active region of the scintillation plate and the photocathode surface. For the detection of higher energy gamma radiation, higher atomic number (i.e., high-Z) materials are commonly used (e.g., LSO, BGO, CsI, etc.), and/or thicker scintillation crystals (e.g., 3 cm instead of 1 cm). In the case of thicker scintillation crystals, the increased thickness reduces image resolution.
Based on the foregoing, there is a need for a radiation sensor with high resolution capability, fast pixel response, minimal dead-time, improved radioisotope identification, and which can be manufactured in large sizes relatively inexpensively.